When firefighters and other emergency personnel arrive at a smoke-filled or burning building or structure, a search and rescue operation is conducted in which each room of the burning structure is systematically searched for persons trapped by the structure fire or otherwise unable to escape from the burning structure. Typically, the firefighters don protective survival gear (e.g., self contained breathing apparatus) and search in teams by following the structural walls. Dense smoke and darkness severely limit the visibility inside the burning structure and hamper firefighter search and rescue operations, sometimes at the risk of the firefighter's safety.
After each room in the burning structure is searched and before searching the next room, a conventional type of marker may be used to mark the entrance leading into the searched room. These markers indicate to subsequent firefighters that the room has been previously searched during the search and rescue operation. Marking searched rooms expedites the search and rescue operation as rooms are not needlessly searched multiple times by different firefighters or by even the same team of firefighters disoriented by the dense smoke and darkness. Searched rooms may be indicated by, for example, placing a chair in the doorway or by marking the door with a crayon or chalk mark.
Another conventional type of marker may be applied to a door at the entrance leading into the room. These conventional markers may rely on a visible indicator, such as a visible strobe light, a reflector, or a colored object, to alert the firefighters that a particular room in the burning structure has been searched. However, smoke has a large component of micron-sized carbon soot particles in it, making it very absorbing at the visible-light wavelengths. Hence, visual indicators are inadequate under conditions of dense smoke and darkness in which vision is obscured. Conventional markers may also rely on an audible signal, such as sound emitted from a speaker, to alert the firefighters that a particular room in the burning structure has been searched. However, the firefighter's ability to discern markers emitting audible signals may be indistinguishable from other environmental noises or may be muffled and muted by the survival gear worn by the firefighter. Verbal communications to communicate searched rooms is also obscured by the environmental noises and the firefighter's survival gear.
Thermal imaging cameras (“TICs”) permit firefighters to penetrate heavy smoke and overcome the handicap of darkness to visualize heat sources in situations of limited visibility during search and rescue operations. Typically, a fireman carries a portable, hand-held thermal imaging camera into a burning structure and relies on thermal patterns visible in the camera display that indicate the presence of a person, a hot spot which may be the source of the fire, or some other thermal characteristic or heat emitting object of interest. The thermal imaging camera converts infrared radiation emitted by the heat source, which is not visible to the human eye, into a visible image viewable on the camera's display. The thermal imaging camera detects the frequency or wavelength of the radiation, which is related to a specific temperature. The wavelength of the emitted infrared radiation increases as the temperature of the heat source increases. On the display of the thermal imaging camera, heat sources are displayed with a color or gray scale in which the displayed brightness increases with temperature. The detection threshold of the electronics in the thermal imaging camera is typically adjusted so that low temperature objects are not visible in the displayed image. This improves the image contrast between heat sources of interest and background objects.
Conventional markers that rely on the emission of visible light or sensing by the unaided human eye are not imaged by the thermal imaging camera as visible wavelengths are outside of the infrared band of the electromagnetic spectrum. Moreover, the temperature of objects emitting visible light lack a sufficient heat signature to be plainly visible in a thermal imaging camera. In addition, the range of the visible light emitted from the object and the ability to see a visual indicator is limited by an inability to penetrate the dense smoke. Conventional markers that rely on reflection are ineffective in the absence of an infrared radiation source for reflection. Thermal imaging cameras are passive devices in that they do not carry such infrared radiation sources. Even if an infrared source were married with a thermal imaging camera, the high directionality of the infrared light beam projected from the camera would dramatically restrict the firefighter's ability to successfully reflect infrared radiation from the reflector. Colored objects cannot be visualized using a thermal imaging camera because of the invisibility of light in the visible band of the electromagnetic spectrum to the camera's imaging system. If firefighters are compelled by the nature of the structure fire to crawl during the search and rescue operation, conventional room markers may be difficult to distinguish visually. Obviously, sound emitting markers cannot be imaged by the thermal imaging camera.
What is needed, therefore, is a marker for a room door that may operate in conjunction with a thermal imaging camera during search and rescue operations conducted by firefighters inside a smoke-filled or burning structure that overcomes these and other deficiencies of conventional markers.